In recent years, a large amount of available spectrums in millimeter-wave frequency band has drawn much attention in meeting the rapidly increasing demand for mobile communications. Channel characteristics in millimeter-wave frequency band are not very suitable for mobile communications due to great path fading, but massive multi-input multi-output technology may effectively compensate for path fading in the millimeter-wave band.
In consideration of the number and the size of antennas and the antenna spacing, for a base station operating in a frequency band from 1 GHz to 4 GHz, the base station will become bloated when it is equipped with a large number of antennas. The size of the base station may be greatly reduced with the usage of millimeter-wave frequency band. For example, since the size of the antenna is proportional to the wavelength of radio wave, the size of an antenna for 60 GHz frequency band is only 1/30 of the size of an antenna for 2 GHz frequency band. The antenna spacing has the similar characteristics. Therefore, a combination of the massive antenna technology and the millimeter-wave communication makes a base station equipped with a large number of antennas possible.
With respect to the base station equipped with a large number of antennas, in a conventional full-digital precoding architecture, baseband data streams for K users are mapped to M radio frequency (RF) chains and antennas through a baseband full-digital precoding matrix, so as to achieve the best precoding performance. However, this structure requires M radio frequency chains, resulting in large demand for millimeter-wave devices and high power consumption.
In order to achieve a trade-off between performance and power consumption, a hybrid precoding architecture is proposed, in which L (where K|L<<M) radio frequency chains are used to connect baseband data streams to antennas via phase shifters. For example, a hybrid precoding architecture with fixed sub-connections is provided in patent application publication No. WO2013119039A1, in which a specific portion of an antenna array is used for signal transmission for a specific radio frequency chain. However, different radio frequency chains cannot share the antennas with each other, which results in poor flexibility in configurations of the radio frequency chains and the antennas.